The epidermal growth factor receptor (EGFR) is upregulated in squamous cell carcinomas of the head and neck (SCCHN) where EGFR expression levels in the tumor correlate with decreased survival. The EGFR monoclonal antibody, cetuximab, was FDA-approved in 2006 for the treatment of SCCHN and is the first new drug for this cancer in 45 years. However, despite the ubiquitous expression of EGFR in SCCHN tumors, the clinical response rate to cetuximab as single agent therapy is limited, underscoring the importance of elucidating the mechanisms of persistent tumor growth in the setting of wild-type EGFR blockade. We have accumulated extensive evidence that GPCR-mediated signaling through EGFR-dependent and EGFR-independent pathways contributes to SCCHN growth and survival. Patients who do not respond to cetuximab are likely to succumb to their head and neck cancer. We currently do not know how to identify SCCHN patients who will benefit from EGFR targeting. Our working hypothesis is that persistent GPCR signaling in the setting of EGFR blockade contributes to tumor progression. Completion of these studies will elucidate mechanisms of resistance to EGFR targeting strategies thus facilitating the design of therapeutic regimens to enhance clinical response. To accomplish the goals of this proposal, we will: 1) determine the GPCR-stimulated pathways in SCCHN that are activated upstream or downstream of EGFR with the goal of identifying therapeutic targets;2) elucidate the EGFR-independent pathways induced by GPCR in SCCHN;and 3) determine the antitumor effects of EGFR targeting in combination with key GPCR signaling intermediates in preclinical SCCHN models. PUBLIC HEALTH RELEVANCE: The proposed studies will elucidate the mechanisms of resistance to EGFR targeting strategies in preclinical models of squamous cell carcinoma of the head and neck. Specifically, we will test the hypothesis that persistent G-protein-coupled receptor (GPCR) signaling in the setting of EGFR blockade contributes to limited clinical responses to EGFR targeting in SCCHN. In addition, we will test the hypothesis that GPCR signaling contributes to cetuximab resistance in 2 SCCHN patient cohorts and that GPCR targeting in combination with EGFR blockade modulates expression of proteins and phosphoproteins in human SCCHN tumors from subjects treated on a clinical protocol.